Kanda has been like this for as long as he can remember. He has no memories of a time when people didn't hate and fear him for what he was, and what he wasn't. He has no memories of a time when he didn't hate and fear them back. They hated his indifference, his strength and his coldness. He had so many memories of people, finders or other exorcists, yelling at him for not caring. When something would go wrong, those times when a bystander was killed in collateral damage on a mission, things like that. Those people would look at him and angrily react to his lack of sympathy, ask him if he even cared. He would honestly answer no. He understood that they were in pain. But he wasn't. There was no point in their wasted energy on all of their emotions, over things that were beyond their control. It was why he hated Finders. Too human, they never seemed to understand that any effort made by them to mourn a lost comrade was futile, and the moment that other person was dead they were forgotten. They had never existed to begin with. Their mothers would receive no ashes.
There is an honest bluntness that he approaches his job with. There is an honest bluntness that he approaches death with. He knows, and has known for as long as he can remember, that any illusions about battles to the death are what get you killed.
He knows that to kill an opponent, you must be at least as strong as, if not stronger than them if you intend to win. He doesn't rely on chance, or another's help, or anything. He relies on himself alone to survive. If he is to ever encounter something that could beat him, he will die. To survive, he makes sure he will never be beaten.
To survive, The Order made sure he would never be beaten. Like a cat, he has more than one life, and he can only be curious so many times, Komui always reminds him. But The Order wanted an Exorcist that wouldn't die, so they created him.
The hard part for The Order it would seem, goes back to the part where you have to be at least strong as your opponent to win. What does one do to make a ten-year-old boy able to survive and win a fight with advanced killing machines?
Akuma are fighting weapons designed for the sole purpose of murder, created for mass death. They are fighting monsters. The Order learned, and he has also learned for himself since, that it takes a monster to kill a monster.
The Order knew that they would need monsters on their side to kill the monsters on the Earl's side. They learned that it would take a monster made by them to kill monsters made by someone else. It is a hard thing to do. Bak still expresses anxiety and regret about what they did. It has always given Kanda an awkward feeling to be around him, because he knows that Bak still feels so much guilt about Kanda's existence. He tries to ignore it, and hopes that Bak will do the same.
But as the monster they have made him, he is one of the most effective exorcists they have. They have made him as strong, as horrifyingly strong and terrible as the monsters he kills and it is the reason he is still alive. It is also the reason he hasn't been alive in years.
When he hears that Allen Walker is turning into a Noah, he gives no outward reaction. He knew his face remained impassive. But he thought on it longer than he cared to admit.
From the day he met the sprout he disliked him. He was cursed, marked as a monster permanently. People have curses for a reason, and Kanda doesn't like a threat. He is blunt about death, and always knows when he can kill someone. He never takes chances. But the kid was and is a chance, it has always been impossible for Kanda to measure just how dangerous he is. He lacked some of his humanity, Kanda could tell. Just behind that hollow smile was the place where that humanity should be. He might not know what humanity would look like, what with the company he keeps, if it weren't for Lenalee. God knows Lavi doesn't have it.
Allen is like Kanda in that lack of humanity. He is unlike Kanda in that he didn't, and still doesn't seem to understand that there is anything wrong with him. He is either unaware or he wants everyone else to be. That kind of deceit is pretty dirty, and Kanda wanted to have nothing to do with it. He made it plain that he would have nothing to do with that boy. He wouldn't shake his cursed hand.
He sees Allen get stronger and stronger, and he knows what is happening. Allen seems willing to rise to any occasion. No matter what, he always seems to come through for them. From killing a Level 2 on their first mission to being able to control the ark, he never stops rising. There is nothing that he will not make himself capable of to do what he must.
He is a self-made monster, and it is his own path that he walks to that end. Kanda is not sad for him.
But when Allen is becoming a Noah, it's not his choice any more. He is a different level of monster entirely, and Kanda is angry with himself immediately.
He surprises himself with what he thinks and feels first, when he hears. He tries to pretend he never thought the thought at all instead of examining it, because the first stupid thing he feels is hope. If the rule is that you must be as much of a monster as the monster you aim to kill then Allen as the monster that he is destined to become may be able to win. May be able to tip the scale just enough that they stand a chance at survival, or even victory.
Kanda hates that he would put that much faith in another person, hates that he might rely on Allen Fucking Walker for victory. If Allen cannot, and Kanda then cannot because he thought Allen would, it would be his own fault for failing.
He had no idea how it would all come about, but as they fight the Level Four amid the wreckage of an orphanage, he thinks he is beginning to see. Allen screams, hurt by his own Innocence. He knows that something has gone wrong when Clown Crown would hurt Allen. Kanda was with him after the seconds it had taken him to fly there, and when he saw Allen Walker's eyes, he was seeing a monster unlike anything he had ever seen there. He is angry, at this, at the Sprout for letting it get this bad. Stupid idiot.
He knows he will see Allen's eyes look dead and hollow again. It wasn't just his smile missing humanity any more. He doesn't know why that bothers him. Sprout's problems should be Sprout's problems. Kanda will never comfort him. What place does a monster have comforting a monster?
The best he can do now is what he has always done. Survive and win. And hope that it is enough.